Scientists may have solved a climate change mystery using Greenland ice cores

Scientists have long known that carbon dioxide
is the main cause of most of the warming we’ve seen since
pre-industrial times. But there are periods in the Earth’s distant
past when the connection between carbon dioxide and temperature
rise has been harder to see.

New
research into Greenland’s ice sheets now seems to have
explained one of the mysteries of our climatic past, confirming the
importance of carbon dioxide on global temperature changes.

Mystery interval

Around 20,000 years ago the Earth was emerging from an ice age
as orbital changes meant it received slightly more of the sun’s
energy.

Scientists
think these changes caused carbon dioxide
from the oceans to be released into the atmosphere.

Yet despite the planet being closer to the sun
and higher levels of carbon dioxide, records for Greenland didn’t
seem to show much change in temperature. While the rest of the
northern hemisphere appeared to warm, Greenland didn’t seem to
follow suit for another 3,000 years. Scientists
couldn’t explain why, and it was even dubbed the ‘mystery
interval’ by one
study.

But now the new
study published in the journal Science
suggests that temperatures actually had
risen – but the rise wasn’t captured by earlier ice core
records.

To understand how temperatures changed thousands
of years ago scientists use a set of techniques called
”palaeoclimatology’ which look for places where climate
information is stored, like tree rings, lake sediments and ice
cores.

Ice cores can be extracted by drilling into
ice sheets. These giant vertical samples contain years and years of
compacted snow in a chronological record –
allowing scientists to reconstruct past temperature and
rainfall.

Previously, scientists had analysed oxygen in
the ice itself to unlock secrets of past climate. This analysis had
led to the conclusions about delayed warming in
Greenland.

But in the new study, scientists from Oregon
State University analysed nitrogen in tiny air bubbles held in the
ice cores instead.

There is an important difference. Analysing the
chemical makeup of ice can be affected by changes to the water
cycle, the same isn’t true for air, allowing for a more accurate
reconstruction of what happened in Greenland, the researchers
say.

Analysis from three ice cores in Greenland
showed not only that temperatures rose along with atmospheric
carbon dioxide during the mystery period, but also that the warming
trend of around 5 °C closely matched what climate models said
should happen.

And the method could be applied in the Antarctic
as well. Dr Louise Sime from the British Antarctic Survey, who
wrote a comment article to go with paper, told us:

“It would be exciting if
similar methods could be applied to the Antarctic ice core record,
since we also lack continuous independent temperature
reconstructions for this region.”

This could help scientists understand how
temperature changes affect ice sheets in Antarctica. Sime says the
method could be used to “pin down temperature changes which might
trigger the loss of the West Antarctic ice sheet.”

In solving the Greenland puzzle, the study
has reinforced in a small way the connection between atmospheric
carbon dioxide and global temperatures, and perhaps serves as a
reminder that even small changes to the Earth’s energy balance can
lead to big changes to our climate.